Usually aircraft tails which are trimmable in its tilt to the aircraft body, are trimmed by the use of trapezoid thread spindles or ballscrews, that is to alter its tilt in relation to the aircraft body. Such actuators used for alteration of the tilt of aircraft tails in relation to the aircraft body are usually called “Trimmable Horizontal Stabilizer Actuator” (short form: THSA) in the field of aerospace technology, which term will be used in the present invention. To hold a load statically, trapezoid thread spindles are constructed as self-locking and ballscrews as retarded.
Though it would be desirable to use pneumatic or hydraulic piston cylinder units as Trimmable Horizontal Stabilizer Actuators, normally this is not possible due to the fact that in case of a malfunction of the pressure feed of such a piston cylinder unit, the load can no longer be held in position. Though final position locking mechanisms like the so-called “Krüger Actuator” are known, such a final position locking mechanism is unable to fix the piston cylinder unit in an arbitrary position. Furthermore there are different known techniques for holding a piston cylinder unit in an arbitrary position, however these systems, as a general rule, are based on a non-positive connection, which is carried out by a friction coupling in the interior of the cylinder, like in the case of the so called “Hähnchen Zylinder” by the company Hänchen.
With such a non-positive locking mechanism, only comparatively small static retention forces can be generated, which are considerably lower than the actuating power created by the piston cylinder unit itself. Thus, the known solutions for locking of a linear actuator can only securely lock the same in one defined position or apply only minor retention forces (like for example with a non-positive locking mechanism).
Furthermore, the described known locking solutions are only poorly or not at all inspectable, as they are built into the interior of the piston cylinder unit. Due to the aforementioned disadvantages, piston cylinder units cannot be considered as Trimmable Horizontal Stabilizer Actuators.